[llvm-dev] Question about Traversing Loops in forward or reverse program order on new pass manager

[Philip Reames via llvm-dev]

On 4/15/21 10:27 AM, Krzysztof Parzyszek via llvm-dev wrote:
>
> I think the traversal order may depend on the optimization.  I would 
> actually propose to get rid of those fine-grained pass managers 
> altogether.  I think they are abstractions taken too far, to the point 
> of being counter-productive.  Instead, loop passes should visit a 
> function at a time, and use utility functions (like iterators of 
> various kinds) to visit loops in the order they want.
>
JFYI, there are very good compile time reasons to visit each loop with 
all passes.  I don't remember the exact numbers, but I remember last 
time this was assessed that locality made a very measurable impact.  
Particularly for large machine generated codebases.

If we do have loop transforms which prefer both, we could do two pass 
approach.  We have something analogous to this for inferring attributes 
in IPO.

> On the actual issue here, I think we should stick to the old traversal 
> order for the time being.  This would reduce the potential for 
> regressions and make it easier to fully transition to the NPM for 
> everyone.
>
> -- 
>
> Krzysztof Parzyszek kparzysz at quicinc.com <mailto:kparzysz at quicinc.com> 
> AI tools development
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of 
> *Arthur Eubanks via llvm-dev
> *Sent:* Thursday, April 15, 2021 11:59 AM
> *To:* Jingu Kang <Jingu.Kang at arm.com>
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* [EXT] Re: [llvm-dev] Question about Traversing Loops in 
> forward or reverse program order on new pass manager
>
> Ping on loop traversal order. Does anybody have any intuition about 
> which way to traverse loops in a function?
>
> On Wed, Apr 7, 2021 at 12:36 PM Jingu Kang via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     Hi All,
>
>     I have seen performance regressions from new pass manager against
>     legacy pass manager. One of the issues is the order of populating
>     loops on pass manager. The legacy pass manager is traversing loops
>     in reverse program order but the new pass manager is traversing in
>     forward program order. It sometimes causes different output. I
>     have created a review https://reviews.llvm.org/D99774
>     <https://reviews.llvm.org/D99774> for reverse program order on new
>     pass manager and had short discussion with Arthur on it. It is not
>     easy to say the reverse order is better than the forward one or
>     vice versa. I would like to share this issue with more people on
>     llvm-dev. If you have idea or experience about this issue, please
>     share it.
>
>     Thanks
>
>     JinGu Kang
>
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